This invention relates to optical scanners in general and more specifically to a light source assembly for a scanner that provides enhanced depth of illumination.
Optical scanners are well-known in the art and produce machine-readable data which are representative of a scanned object, e.g. a page of printed text. Most optical scanners employ line-focus systems in which light from an illuminated scan line on the object is imaged by a lens onto a linear photosensor array or detector positioned remotely from the object. The linear photosensor array is typically a single dimension array of photoelements that correspond to small area locations along the illuminated scan line. These small area locations are commonly referred to as "picture elements" or "pixels." Each photoelement produces a data signal that is representative of the intensity of light from the corresponding pixel. The data signals from the photoelements are received and processed by an appropriate data processing system which may subsequently store the data on a suitable medium or generate a display signal therefrom for reproducing an image of the object with a display device such as a CRT or a printer.
Optical scanners and various components thereof are disclosed in U.S. Pat. Nos. 4,926,041 for OPTICAL SCANNER of David Wayne Boyd; 4,709,144 for COLOR IMAGER UTILIZING NOVEL TRICHROMATIC BEAM SPLITTER AND PHOTOSENSOR of Kent J. Vincent; 4,870,268 for COLOR COMBINER AND SEPARATOR AND IMPLEMENTATIONS of Kent J. Vincent and Hans D. Neuman; 5,038,028 for OPTICAL SCANNER APERTURE AND LIGHT SOURCE ASSEMBLY of Boyd, et al.; and 5,227,620 for APPARATUS FOR ASSEMBLING COMPONENTS OF COLOR OPTICAL SCANNERS of Elder, et al., which are each hereby specifically incorporated by reference for all that is disclosed therein.
While optical scanners of the type described above are useful for scanning objects that can be placed flat on the platen glass of the scanner, such as pages of printed text, there are instances where it is desirable to scan objects that cannot be made to lay completely flat, such as the pages of an opened hardbound book, or other objects that cannot be made to lay flat on the platen glass of the scanner. If such objects are to be scanned, the optical system of the scanner should be provided with a large depth of field so that the images of those points that do not lay directly on the platen glass will also be focused on the detector. Used in this sense, the term "depth of field" is defined as the distance between a near limit focal plane and a far limit focal plane; the images of those points on an object that are located between the near and far limit focal planes being substantially focused on the detector.
While scanners with optical systems having such large depths of field are known, it is difficult to provide an illumination system that can properly illuminate all points on the object that lay within such a large depth of field. Consider, for example, an illuminated scan line S on an object O, shown in FIG. 1, the image of which is focused onto the surface of a detector D by a lens L. In order for the detector D to capture the desired detail of the object O along the entire scan line S, all portions of the illuminated object O in the scan line S should equally illuminate the surface of the detector D. Unfortunately, however, the illumination, i.e., the density of luminous flux, on the surface of detector D varies inversely with the square of the distance from the illuminated object O (a luminous source) and directly with the cosine of the angle .theta. between the luminous flux and the normal N to the surface of detector D. Therefore, the illumination on the surface of the detector D decreases rapidly towards each end, and some means for compensating for this reduction in illumination must be found if the detector is to have the same effective sensitivity for points near each end of the scan line as it does for points near the center of the scan line.
The illumination problems described above tend to be even more severe when scanning objects within a relatively large depth of field. Consider, for example, those points on the object that are located near the far limit focal plane of an optical system ("far points"). Not only are such "far points" farther from the detector, they are also usually farther from the illuminating light source as well. Of course, points that are farther from the light source will receive less illumination in accordance with the inverse square law described above. Therefore, the illumination loss at the detector when scanning objects within a relatively large depth of field is due to two factors: 1) the increased distance that such "far points" are located from the detector; and 2) the fact that such far points receive less illumination from the light source in the first place. All in all, the illumination problems associated with scanning objects within a relatively large depth of field tend to be the most severe for those points on the object that are located near the ends of the scan line and also near the far limit focal plane.
Another problem stems from the fact that the fluorescent lamps commonly used to illuminate the object being scanned are usually subject to intensity variations along their lengths. For example, it is common for a fluorescent lamp to produce light of a greater intensity at points near the center of the lamp than at points near the ends of the lamp. Therefore, while it may be possible to use such a fluorescent lamp to provide a satisfactory depth of illumination for points near the center of the scan line, the decreased intensity near the ends of the lamp only aggravates the problem of illuminating sufficiently those points located near the ends of the scan line and/or located near the far limit focal plane, as described above.
Therefore, there remains a need for an optical scanner having enhanced depth of illumination to increase the effective depth of field of the scanner. For good scanner performance, the illumination should be substantially uniform across the entire depth of field, that is, the illumination provided to the "far points" on the object (i.e., those points located at about the far limit focal plane of the scanner optical system) should be sufficient to allow them to be detected by the detector assembly. At the same time, however, the illumination provided to the "near points" on the object (i.e., those points located at about the near limit focal plane) should not be so great so as to adversely affect scanner performance or overload or "swamp" the detector. The enhanced illumination should also be uniform along the length of the scan line, so that the effective depth of field of the scanner is substantially uniform along the entire length of the scan line. Ideally, the depth of illumination should also correspond roughly to the depth of field of the scanner optical system.